1. Field of the Invention
The present invention relates to a technique suitable for being applied to a position detector, a position detecting circuit, and a position detecting method.
More particularly, the present invention relates to a technique for reducing noises immixed from a position detection planar surface in a position detector using an electrostatic capacitance system, thereby enhancing a detection precision for a finger of the human body.
2. Description of the Related Art
There are known various kinds of input devices each of which gives position information to a computer. Of these input devices, there is known a two-dimensional position information input device (hereinafter referred to as “a position detector” for short) called a touch panel.
A touch panel is an input device such that a detection planar surface is touched with either a finger or an input device such as a dedicated pen, thereby carrying out a desired input manipulation for the computer or the like. Thus, with the touch panel, a position where either the finger or the dedicated pen touches is detected to specify a position on a screen, thereby issuing an instruction corresponding to the position where either the finger or the dedicated pen touches to the computer.
The touch panel is commonly used in a Personal Digital Assistant (PDA), an Automated Teller Machine (ATM) in a bank, a ticket-vending machine in a station, or the like.
There are known various kinds of position information detecting techniques each adopted in the touch panel. For example, there are known the position information detecting technique, using a resistance film system, for detecting a position in accordance with a change in pressure applied on a detection planar surface, the position information detecting technique, using an electrostatic capacitance system, for detecting a position in accordance with a change in electrostatic capacitance of a film on a surface of a detection planar surface, and so on.
The principles of an operation of the position detector using the electrostatic capacitance system will be described below.
Electrode lines are wired on each of a front surface and a back surface of an insulating sheet having a rectangular shape or the like. In this case, the electrode lines on the front surface of the insulating sheet, and the electrode lines on the back surface of the insulating sheet are disposed in a matrix as a whole. An AC signal is supplied to the electrode lines wired on one surface of the insulating sheet, and a current is detected from the electrode lines wired on the other surface of the insulating sheet. Capacitors are formed in respective intersection points between the electrode lines wired on the one surface of the insulating sheet, and the electrode lines wired on the other surface of the insulating sheet. Thus, when an AC voltage is applied across the electrode lines of the one surface of the insulating sheet, and the electrode lines of the other surface of the insulating lines, a current is caused to flow through each of the intersection points of the insulating sheet.
At this time, when a frequency of an AC signal is set, for example, at 200 kHz, and a finger of the human body is made to come close to the electrode lines wired on one side of the insulating sheet, there is caused a phenomenon that a part of the electric charges accumulated in the capacitor by applying the AC voltage across the capacitor is absorbed by the finger of the human body. A change in electrostatic capacitance of the capacitor caused by the absorption of the part of the electric charges in the finger of the human body is detected in accordance with the current caused to flow through the capacitor concerned. However, since the current capable of being detected is very weak, the current is converted into a voltage signal by a current-to-voltage converting circuit composed of a well-known operational amplifier, and a resulting voltage signal is then amplified.
The prior art related to the present invention is described, for example, in Japanese Patent Laid-Open No. Hei 10-020992.